Giant axons in the crayfish and squid have been studied. In both systems evidence for the substantial transfer of newly synthesized macromolecules (proteins) from Schwann cells (glia) into axons has been obtained. In the case of crayfish motoraxons, the glia appear to be a sufficient source of axonal proteins to maintain the axon (including transmitter enzymes). The proteins on the inner surface of the squid giant axon membrane have been studied using membrane labeling techniques. A correlation between the presence of specific proteins on the membrane and excitability of the axon has been found.